Gear mechanisms, such as a trapezoidal thread worm gear mechanism or a rack and pinion gear mechanism, have generally been used as mechanisms to convert the rotary motion of an electric motor into the axial linear motion in an electric linear actuator, used in various types of driving sections. These motion converting mechanisms involve sliding contact portions. Thus, power loss is increased. Accordingly, the size of electric motor and power consumption are increased. Thus, ball screw mechanisms have been widely used as more efficient actuators.
In prior art electric linear actuators, an output member, forming a ball screw, can be axially displaced by rotating a nut, with an electric motor supported on a housing of the actuator, to axially drive a ball screw shaft inserted into the nut. The electric linear actuator (power transmitting apparatus) 51 shown in FIG. 5 is a type used in an electric braking apparatus. It has a power drive transmitting mechanism 55 including a gear mechanism (speed reduction mechanism) 52, a ball screw mechanism 54 and a ball screw shaft 53. The gear mechanism 52 transmits rotational power of an electric motor (not shown) to the ball screw mechanism 54. This converts the rotational power to linear driving power of a ball screw shaft 53.
A housing 56 contains the driving power transmitting mechanism 55 that includes the gear mechanism 52 and the ball screw mechanism 54. The housing 56 has a case 56a and a cover 56b. It is formed from light metal such as aluminum alloy etc. The case 56a includes a plurality of screw apertures 57 to mount the electric motor to the driving power transmitting mechanism 55.
The gear mechanism 52 includes a pinion gear (not shown) secured on an output shaft of the electric motor. An idler gear 58 mates with the pinion gear and a ring gear 59 mates with the idler gear 58.
The ball screw mechanism 54 includes a nut 60 rotated by the rotational driving power from the electric motor through the gear mechanism 52. A ball screw shaft 53 is axially moved by the nut 60 through balls (not shown) rollably arranged between screw grooves of the nut 60 and the ball screw shaft 53.
The case 56a and cover 56b are separate structures. The case 56a includes a plurality of through apertures 62 that receive bolts 61. The bolts 61 are screwed into a plurality of threaded apertures 63 formed on the cover 56b, at positions corresponding to the through apertures 62. Thus, the case 56a and cover 56b can be united by inserting the bolts 61 through the apertures 62 and fastening them to the threaded apertures 63.
A bearing 64 rotationally supports a tip end of an output shaft of the electric motor. The tip end is mounted in a bore 65 formed on the top of the cover 56b. A pin 66, to prevent rotation of the ball screw shaft 53, is vertically mounted on the ball screw shaft 53. Thus, the pin 66 can be received in a slide groove 67 and linearly guide the ball screw shaft 53 along the slide groove 67. See, JP 2012-214090 A.
In the prior art electric linear actuator 51, the gear mechanism 52 and the ball screw mechanism 54 are contained within a two-piece type housing 56. The electric motor is mounted outside the housing 56. Its rotational power is transmitted to the ball screw mechanism 54 via the gear mechanism 52 which, in turn, is converted into linear motion of the ball screw shaft 53.
In the electric linear actuator 51, it is possible to reduce the diameter of the gear mechanism 52 since the motor shaft and the gear mechanism 52 are connected via the idler gear 58. However, to achieve a compact configuration of the housing 56, an outline configuration of the housing 56 is adopted with a pear-like shape partially projecting with the top portion of the cover 56b. 
In such an electric linear actuator 51, driving parts, such as the ball screw mechanism 54, are contained within the housing 56 to prevent entry of muddy water or oil into the driving parts or scattering of lubricant grease to the outside. The housing 56 should have sufficient strength and durability to withstand loads applied from the driving parts and to prevent deformation of abutment surfaces or fitting surfaces between the casing 56a and cover 56b caused by reaction forces of the nut 60 against propelling forces of the ball screw mechanism 54. In this case, since the configuration of the fitting surfaces of the case 56a and the cover 56b is noncircular (i.e. pear-like configuration), it causes uneven stress therebetween and accordingly impairs sealability of the housing 56.
To reduce manufacturing cost of the electric linear actuator 51, it is preferable to omit the idler gear 58 and fastening bolts 61. However, omission of the idler gear 58 will enlarge the diameter of the gear mechanism 52. Omission of the bolts 61 will impair sealability of the housing 56.